Regardless of the design of a combustion engine (for example reciprocating engine, pistonless rotary engine or free-piston engine), noises are generated because of the working cycles (in particular sucking-in and compressing a fuel-air mixture, ignition/expansion and exhausting the combusted fuel-air mixture) taking place in succession. These noises on the one hand pass through the combustion engine as solid-borne sound and are radiated off the outside of the combustion engine as airborne sound. On the other hand, the noises pass through an exhaust system of the combustion engine as airborne sound together with the combusted fuel-air mixture.
These noises are frequently perceived disadvantageous. On the one hand, there are legal stipulations for noise control, which have to be adhered to by manufacturers of vehicles driven by combustion engines. These legal stipulations as a rule specify a maximum permissible sound pressure during the operation of the vehicle. On the other hand, manufacturers attempt to impart a characteristic sound development on the combustion engine driven vehicles produced by them, which fit the image of the respective manufacturer and are to appeal to the customers. With modern engines including low cubic capacity, this characteristic noise development can frequently no longer be ensured in a natural way.
The noises passing through the combustion engine as solid-borne sound can be dampened with high efficiency and therefore usually do not constitute a problem regarding noise control.
The noises passing through an exhaust system of the combustion engine as airborne sound together with the combusted fuel-air mixture are conventionally reduced by silencers/mufflers arranged before the mouth of the exhaust system. If applicable, the silencers/mufflers are arranged downstream of existing catalytic converters. Such silencers/mufflers can for example operate according to the absorption and/or reflection principle. Silencers/mufflers constructed in accordance with either of these principles have the disadvantage that they require a comparatively large volume and put up a relatively high resistance to the combusted fuel-air mixture in case a high damping efficiency is required. Consequently, by using silencers/mufflers constructed in accordance with either of these principles the overall efficiency of the vehicle drops and the fuel consumption rises.
As alternative or to complement silencers/mufflers, so-called active noise control systems have been in development for some time, which superimpose electro acoustically generated anti-sound (sound wave with the same amplitude but with inverted phase (antiphase) to the noise to be cancelled) on the airborne sound generated by the combustion engine and conducted in the exhaust system. Such systems are known for example from the documents U.S. Pat. No. 4,177,874, U.S. Pat. No. 5,229,556, U.S. Pat. No. 5,233,137, U.S. Pat. No. 5,343,533, U.S. Pat. No. 5,336,856, U.S. Pat. No. 5,432,857, U.S. Pat. No. 5,600,106, U.S. Pat. No. 5,619,020, EP 0 373 188, EP 0 674 097, EP 0 755 045, EP 0 916 817, EP 1 055 804, EP 1 627 996, DE 197 51 596, DE 10 2006 042 224, DE 10 2008 018 085, DE 10 2009 031 848. By using an active noise control system as alternative or to complement silencers/mufflers, the construction volume of an exhaust system can be reduced by up to 60%, the weight can be reduced by up to 40% and the exhaust back pressure can be reduced by up to 150 mbar. The term anti-sound serves to distinguish from the airborne sound (noise) conducted in the exhaust system. Considered on its own, anti-sound is conventional airborne sound with the same amplitude but with inverted phase to the original sound (noise) to be cancelled.
A corresponding active noise control system is shown in the FIGS. 1 and 2 and can be procured from the company J. Eberspächer GmbH & Co. KG, Eberspächerstrasse 24, 73730 Esslingen, Germany.
FIG. 1 schematically shows a perspective view and FIG. 2 a block diagram of an active noise control system connected to an exhaust line.
As is evident from FIG. 1, both a sound generator 3′ of an active noise control system as well as an exhaust pipe 4′ fluidically connected to a combustion engine (in fluid communication with a combustion engine) lead into a tailpipe 1′ in the region of an orifice 2′ of an exhaust system. In the tailpipe 1′, the airborne sound conducted in the exhaust pipe 4′ together with the combusted fuel-air mixture is superimposed by anti-sound generated in the sound generator 3′ of the active noise control system. To verify the effectiveness of the anti-sound, the tailpipe 1′ comprises an error microphone 5′.
As is evident from FIG. 2, a catalytic converter 7′ is provided between the combustion engine 6′ and the exhaust pipe 4′. In addition, a temperature probe 9′ connected to an engine control 8′ for determining the exhaust gas temperature is arranged in the exhaust pipe 4′. The engine control 8′ is connected to the combustion engine 6′. In the engine control 8′, an anti-sound control 10′ is integrated, which is connected to the error microphone 5′ of the tailpipe 1′ and to a loudspeaker 12′ belonging to the sound generator 3′ via an amplifier 11′.
For achieving a destructive interference of the sound waves of the airborne sound conducted in the exhaust pipe 4′ and of the anti-sound generated in the sound generator 3′ in the region of the tailpipe 1′, the sound waves in the tailpipe 1′ originating from the sound generator 3′ have to correspond in shape and amount, to the sound waves conducted in the exhaust pipe 4′, but have a phase shift of 180 degrees (inverted phase) relative to these. For controlling the loudspeaker 12′, the anti-sound control 10′ makes use of empirically (experimentally) determined characteristic curves, which take into account the transmission distance between loudspeaker 12′ of the sound generator 3′ and the error microphone 5′ in the tailpipe 1′ and indicate the signal to be output to the loudspeaker 12′ as a function of a rotational speed of the combustion engine 6′ received from the engine control 8′. Since the propagation velocity of sound between loudspeaker 12′ and error microphone 5′ is temperature-dependent, the characteristic curves are also temperature-dependent and thus only suitable for a defined (nominal) temperature range. The selection of the characteristic curve applicable to a temperature range is made by the anti-sound control 10′ by means of the value measured by the temperature sensor 9′.
Thus, the anti-sound control 10′ selects a characteristic curve that is suitable for this temperature range as a function of a value measured by the temperature sensor 9′, from these characteristic curves, reads out values belonging to a respective engine rotational speed and by means of these values, outputs a corresponding signal to the loudspeaker 12′ via the amplifier 11′. The success of the destructive sound wave superimposition is captured with the help of the error microphone 5′.
In the case of known active noise control systems it is disadvantageous that the compensation of the temperature dependency of the speed of sound between loudspeaker and error microphone is complicated.